Development processes by electrophotography include a two-component development process using a developer made of two components, i.e., toner particles and carrier, such as magnetic brush process and cascade process. In general, such a two-component development process developer is a mixture of a toner made of fine particles and a carrier made of particles having a greater size. Due to electrostatic charge having opposing polarities developed by the contact of these particles, when a developer having toner particles retained on the surface of a carrier comes in contact with an electrostatic image on the photoreceptor, the toner particles are attracted by the electrostatic image to form a visible image. The visible image thus formed is transferred to an image support such as paper, and then fixed thereto under heating or pressure.
The quality of the electrostatic image thus formed (image quality) depends on the triboelectricity and resistivity of the carrier and toner, particularly on the material of the carrier core material and the core coating resin layer. In general, as the triboelectricity of the carrier increases, the image density decreases. Further, the carrier can be more easily attracted by the photoreceptor, causing image defects. On the other hand, if the triboelectricity of the carrier is reduced, fog or stain in the interior of the copying machine due to toner scattering can occur more. Further, if the resistivity of the carrier is too high, the resulting image density is limited to a low level or gradually decreases or some edge effect can occur. Moreover, if the resistivity of the carrier is too low, a high image density can be obtained but further generation of fog, reduction of gradation or toner stain in the interior of the copying machine can easily occur.
The optimization of the triboelectricity and resistivity of the carrier has heretofore been accomplished by forming the carrier core material by iron, ferrite, magnetite, hematite or the like or by forming the carrier core coating resin layer by a silicone resin, acrylic resin, polyolefin resin, vinyl resin, polyvinylidene resin, fluorocarbon resin, polyamide resin, polyester resin, polyurethane resin, polycarbonate resin, phenolic resin, melamine resin, amino resin, epoxy resin or the like and incorporating an electrically conductive powder such as carbon black and organic tin compound in the coating layer on the carrier core material or by changing the thickness of the coating layer. However, even if these factors are merely combined, an image with a high density, little fog and a high gradation which are well balanced cannot be easily obtained. Thus, the foregoing approach is disadvantageous.